1. Field of the Invention
The present invention is concerned with method and apparatus for monitoring the rotational speed of the shaft of a gas turbine, having a number of spaced blades on or rotating with the shaft. In particular, the invention is concerned with monitoring the speed of gas turbines having blades mounted such that the blade tips can move relative to each other. Embodiments of the invention are concerned with methods and apparatus for processing signals in order to calculate, with high accuracy and a fast response time, the rotational speed of a gas turbine shaft.
Particular preferred embodiments of the invention that are described below in more detail may be used to generate a sequence of outputs or signals which represent the rotational speed of a gas turbine shaft. Preferred embodiments of the invention easily and effectively compensate for blade movement relative to the shaft, and missing signals or gas turbine blades.
The rotational speed of, for example, a shaft such as a gas turbine shaft may be determined by measuring the time intervals between successive evenly spaced projections rotating with the shaft as they pass a single measuring point or sensor. If the spacing between projections is known, the speed is easily determined from the time or period between the passage of successive projections past the sensor or measuring point.
2. Discussion of Prior Art
The speed of a rotating gas turbine shaft is typically monitored by monitoring the movement of a metallic toothed phonic or tone wheel which rotates with the gas turbine shaft. A magnetic speed probe monitors the changes in a magnetic field as a tooth passes through it. The passage of each tooth generates a probe signal pulse or peak and the probe signal train is used to calculate the rotational speed of the toothed wheel by measuring the time between successive pulses, or counting a number of pulses in a fixed time. The rotational speed of the gas turbine shaft is then derived from the speed of the phonic or tone wheel.
Magnetic variable reluctance sensors (including transformer probe sensors such as that disclosed in EP 169,670) can be used to monitor the movement of a phonic wheel and therefore the rotational speed of a rotating shaft coupled to the phonic wheel.
There is no easy access to the turbine shaft, so the toothed or phonic wheel is typically at a distance from the shaft and connected thereto via a long gear train. A big disadvantage of such a system is that the gear train is expensive and heavy, and can only be replaced during a major engine overhaul. An alternative to the remote phonic wheel coupled to the turbine shaft by a gear train is to mount the phonic wheel directly on the shaft. However this requires additional space inside the engine for the wheel and probe fixture.
Eddy current sensors such as that disclosed in GB 2,265,221 can also be mounted on the outside of an engine and used to measure the rotational speed of a gas turbine shaft by directly monitoring movement of the blades mounted on the rotating shaft. If the separation between blades is known, then the rotational speed can be determined from the time between successive signal pulses where each signal pulse or peak corresponds to passage of a blade past the sensor.
Patent numbers GB 2,265,221, GB 2,223,103, U.S. Pat. No. 3,984,713 and GB 1,386,035 each describe eddy current or inductive sensors which may be used to measure the rotational speed of a bladed shaft. The sensors described in these documents are speed or torque sensors, each comprising a magnet positioned so that the tips of the blades pass in close proximity to the magnet. When a blade is moving close to the sensor magnet, eddy currents are generated in the tip of the blade. These eddy currents generate their own magnetic fields which are detected by a coil located in the sensor. A rotating shaft with blades, such as that in a gas turbine, will therefore generate a series of pulses with the period between pulses representing the period between successive blades as they pass the sensor. The series of pulses can be used to determine the speed of the rotating shaft; the speed is calculated from the time measurements between the pulses or from the time it takes for a pre-determined number of blades to pass the sensor.
It is possible to use other types of sensors to monitor movement of the turbine blades themselves past a measuring point, such as optical, capacitative or RF (radio frequency) sensors, but these sensors cannot operate through the turbine casing and require direct access to the blades through a hole in the casing.
Determining speed from measurements of the time taken for successive blades to past a measuring point (i.e. the blade period) requires the distance between blades to be known. The blades are nominally or theoretically evenly spaced and hence the distance is known. However, as discussed in more detail below, in reality the distance between the blades of a gas turbine is not the same for all the blades of a moving gas turbine, and the distance between two blades can and does vary as the gas turbine rotates.
A method of calculating shaft speed by measuring a time interval between consecutive blades passing a single sensor has been described in GB 2,414,300. The inventors of the subject application are the first to realise that the method described in GB 2,414,300 is not suitable for shaft speed measurement when a fast response time is required. Effective operation of the predictor-limiter method described in GB 2,414,300 requires processing of time information from several time intervals and, especially when the rotational speed of the shaft is low, the response time of the system can be very slow. This is clearly problematic in applications where response time and accuracy are critical.